It is known, for example from U.S. Pat. No. 4,747,051 to compare desired and actual values of the vehicle speed and to use the result of the comparison to control, via a PI-controller, the amount of fuel supplied to the engine by appropriate adjustable supply means. Such arrangements suffer from a number of disadvantages. For example, the means for sensing the actual road speed is frequently inaccurate. Moreover, the actual vehicle speed changes only relatively slowly in response to changes in the amount of fuel supplied.